Activated or adsorbent carbons are solid adsorbents with extremely high internal surface areas. They are produced from a range of carbonaceous starting materials and by different processing routes, and are used in applications that involve purification and chemical recovery. For example, activated carbons may be utilized in various applications to treat waste water, to recover solvents from process streams, to purify air and gases, to reduce gasoline vapor emissions and in gold recovery. Activated carbons may also be utilized in more specialized applications in many instances by the addition of impregnants. For example, impregnated activated carbons are utilized in catalysis, medicine and as purification agents in military and industrial gas masks.
The pore structure of an activated carbon determines its capacity to adsorb molecules of different shapes and sizes. Microporous carbons (i.e. carbons in which the majority of the pores have diameters less than 2 nm) are most effective for the adsorption of small molecules while mesopore carbons (i.e. carbons in which a large proportion of pores have diameters in the range 2-50 nm) are used for the adsorption of large molecules such as color bodies. As carbon surfaces are generally hydrophobic, they are not ideally suited for the adsorption of polar compounds, such as for the removal of phenol from water. The introduction of surface functionalities can, however, enhance the attraction for such molecules. Accordingly, it should be appreciated that the ability to control pore structure and surface chemistry unlocks the door to the utilization of activated carbons in many more technologies and applications involving adsorption, separation and catalysis.
Mechanical strength can also be an important property, especially for granular carbons. More specifically, the carbons must allow handling such as necessary to deliver and pack absorber beds with the generation of a minimal number of fines. More specifically, in many applications the carbon is subjected to repeated movement and must bear the overburden of material when packed in deep beds. For example, in hydrocarbon vapor recovery systems known as evaporative loss control devices (ELCD's), the activated carbon first adsorbs hydrocarbons from an air/vapor stream. Once the adsorption capacity of the activated carbon is reached, the carbon is regenerated under vacuum with the passage of purge air. In contrast, in industrial processes such as polymer processing, regeneration of the carbon may be achieved by steam stripping. The hydrocarbons are then recovered from the effluent condensed stream. Under any such operating conditions (i.e. possibly pressure, temperature, and flow changes), high hardness and low friability (e.g. for dust control) are important properties. Consequently, it should be appreciated that many applications require the activated carbons to have a minimum strength.
Activated carbons in the form of granules, extrudates, powders, fibers or beads, can be produced from suitable thermosetting precursors by either thermal or chemical routes. Common commercial feedstocks include biomass material such as wood, coconut shell and fruit pits, and fossilized plant matter such as peats, lignites and all ranks of coal. The thermal route involves carbonization to about 600-650.degree. C., followed by partial gasification (activation) in steam or carbon dioxide at 800-900.degree. C. to develop the desired pore structure. Chemical activation involves reaction with a reagent such as zinc chloride or phosphoric acid followed by heat treatment to temperatures in the range 400-900.degree. C. An important additional step is leaching to recover residual reagent for eventual recycle.
While the properties of activated carbons are influenced to some extent by the processing route and conditions, the precursor structure is the single most determinant factor. Attempts to modify the porosity or surface chemistry of activated carbons have involved the reaction of the as-formed carbon with agents that can lead either to the deposition of carbon (i.e to narrow the pore dimensions), or the introduction of surface functionalities (i.e. to enhance the attraction of certain molecules to the activated carbons). In either case, the attainment of an uniform distribution is difficult and complicated by restricted access and diffusion of the reactant through the pore structure. The underlying premise of the present invention is that the porosity and surface chemistry of the carbons are defamed and controlled in the synthesis process.